1. Field of the Invention
The present invention relates to an image projecting apparatus having focal-point detecting means, and more particularly to an image projecting apparatus having focal-point detecting means, for example, suitable for an apparatus such as a microfilm reader or a microfilm reader-printer. The apparatus is arranged to detect an image-forming state of a projection image projected onto a screen surface or onto a surface of a recording medium through a projection lens, thereby obtaining an excellent projection image always in focus.
2. Related Background Art
Ordinary image projecting apparatuses such as microfilm reader-printers have a reader portion (observing system) for magnification-projecting an image (image information) of a document onto microfilm or the like, in which demagnified images of documents are recorded onto the screen surface through the projection lens. A printer portion (recording system) is also for projecting the image of the document on a photosensitive member through the projection lens to output a copy thereof.
In many microfilm reader-printers an operator manually moves the projection lens along the optical axis thereof to obtain an in-focus projection image on the screen surface or the surface of photosensitive member.
Because of such an operation, focus adjustment is troublesome, a recent demand is to achieve a reader-printer that can be automatically focused within a short time using focal-point detecting means.
Generally, when a projection magnification is as high as 40 to 50 for the projection lens in the image projecting apparatus, the depth of focus becomes as shallow as 0.02 mm. Thus, the focal-point detecting means is required to have high optical accuracy in order to form a sharp projection image on the screen surface or the surface of photosensitive drum.
A variety of image projection apparatus using the focal-point detecting means are proposed, for example, in Japanese Laid-Open Patent Application No. 63-316838 and No. 63-70812.
In the Japanese Laid-Open Patent Application No. 63-316838, there is provided a reader system optical path for projecting a magnified image and a focus detection optical path for guiding a light beam to a light-receiving element for focus detection. A secondary image-forming lens is disposed in the focus detection optical path to demagnify the projection image of the projection lens to form a demagnified image thereof on the light-receiving element. Then focus adjustment is performed based on a signal obtained from the light-receiving element.
In Japanese Laid-Open Patent Application No. 63-70812, an image sensor is provided for reading a projection image, wherein contrast signals concerning the projection image are obtained in a plurality of regions in the image sensor, and an in-focus position for each region is obtained at a position on the optical axis of projection lens where each contrast signal becomes maximum. Then focus adjustment is made by determining the in-focus position of the projection lens using a set of in-focus positions most frequently appearing among the in-focus positions of the respective regions and located close to each other.
Many of recent microfilm reader-printers are equipped with a rotation prism of a trapezoid cross section on the exit side of a taking lens. An in-plane angle of the projection image on the screen surface or the surface of photosensitive drum is adjusted by rotating the rotation prism with the optical axis thereof as a rotation axis.
Since the rotation prism requires an effective size for sufficiently transmitting the projection light beam from the projection lens, it was apt to become relatively larger in scale. Thus, the conventional apparatus employs such an arrangement that an incident point 8a of the projection optical axis 7a into the rotation prism 8 is shifted relative to an emergent point 8b therefrom as shown in FIG. 2. That is, the two optical axes deviate from each other, thereby decreasing the size of the rotation prism.
In FIG. 2, reference numeral 6 designates an image of a document on a microfilm or the like, 7 refers to the projection lens, and 11 designates the screen surface. Further, .DELTA.h represents an amount of deviation between the entrance-side optical axis and the exit-side optical axis of the rotation prism 8.
In the structure of the deviating optical axes of the rotation prism, however, when the entrance-side optical axis of the rotation prism rotates around the optical axis the position of the exit-side optical axis changes as a circle with the deviation amount .DELTA.h as a radius on the exit side of the rotation prism.
This causes such a problem that, for example, where a fixed focal-point detecting system is set on the exit side of rotation prism, an incident light beam into the focal-point detecting system changes with rotation of the rotation prism, spoiling the focus detection condition.